1. Field of the Invention
The subject invention relates to information recording and reproduction and, more specifically, to methods and apparatus for driving information carrier tape, such as magnetic recording tape, and to tape transports.
2. Prior-Art Statement
The last two decades have seen unprecedented advances in the information recording and reproduction arts, and particularly in magnetic tape recording and playback.
Yet even in the most advanced prior-art tape recording systems, the tape itself has remained a troublesome and largely intractable component. In particular, the tape, such as used in magnetic tape recorders, is a compliant medium with numerous variables. For instance, the tape has a high modulus of elasticity and a thin base that will easily stretch under tension. In the manner of a transmission line with uniform distribution of mass and compliance, a recording tape is subject to various resonance effects. These include longitudinal resonance in a free tape span, transverse or low frequency resonance and a host of other resonance disturbances stemming from the fact that short spans of wider tapes in instrumentation recorders and other high-grade equipment act in effect as membranes with free edges, provoking a multitude of elusive vibration modes.
In practice, these vibration and resonance effects cause flutter which impairs not only the recording process and the resulting recording, but degrades also the reproduction of recordings.
Another troublesome source of flutter is the stickslip friction which causes the tape to move with a jerky motion at very low speeds, where the static coefficient of friction is larger than the dynamic coefficient at capstans, heads and possible other tape-transport interfaces and which in effect initiates or plucks resonance effects.
Further tension variation and similar disturbance effects are in a tape transport caused by such factors as changes in rotational speed of the tape drive, periodic variations from eccentric drive components, mechanical resonances in shafts, mechanical resonances in head mounts, tension disturbances from the tape reel system and external vibrations. In consequence, the use of servo systems, aiming primarily at a reduction of a ratio of flutter to circular frequency called "time base error," has become routine in instrumentation tape recorders, video tape recorders and other high-grade recording/playback machines.
Parallel to this effort has been the emergence of a dual capstan system in which recording, playback and erasing heads engage the tape at a free span between two spaced capstans. Each capstan has a pinch roller associated therewith which, in the nip or contact area between the pinch roller and capstan shaft, clamps the tape for the transfer of motive power thereto.
One of the capstans thereby acts as supply capstan, while the other operates as takeup capstan. In bidirectional drives, the two capstans alternate in these roles.
Unfortunately, true tape speed in a pinch roller system is not equal to the surface speed of the capstan, since pinch rollers typically are of a compliant kind, being applied to the tape at the capstan with a sufficient amount of pressure to provide the requisite friction at the nip for the desired tape advance. In particular, the pinch roller pressure causes an indentation in the compliant pinch roller whereby the effective radius of the roller is reduced, causing the tape to experience a speed variation as between its entry and exit from the nip. As a result, the tape speed is practically determined by the pinch roller, rather than by the capstan. In addition, pinch rollers add considerable inertia to capstan servo systems, thus impeding servo bandwidth, and are a frequent source of trouble in terms of difficulty of alignment, exposure to dust and other contaminants and subjection to bearing fatigue and other durability limiting factors.
These negative aspects have led to the development of a pure friction drive known as "capstan wrapping" wherein the tape extends around part of the capstan at a sufficient wrap angle to generate the requisite drag force on the tape.
Various systems for achieving the requisite wrap angle have existed. For instance, two spaced idler rollers have been employed to arrange the tape in a U-shaped tape path, with a single capstan being located at and forming the bight portion of the tape. In principle, such an arrangement permits the recording and playback heads to be distributed over both legs of the U-shaped tape path. In practice, however, this advantage is rendered largely illusory by the fact that the rotating capstan tends to create a larger tape tension in the upstream leg of the tape path than in the downstream leg.
The same observation practically applies to a variation wherein a dampened idler is situated between, and in contact with, both legs of the U-shaped tape path. In this respect it may be generally noted that dampened idlers, while addressing themselves to a natural desire to reduce flutter generating tape vibration, exact the often exorbitant price of servo bandwidth reduction.
A somewhat more promising approach evolved from the positioning of an idler roller at the bight portion and the location of the capstan at a distance from such idler between the leg portions of the U-shaped tape path.
In an effort to improve requisite tape tension magnitude and equality in that type of closed loop system, the uniform diameter capstan was replaced by a dual diameter capstan between the leg portions of the tape path, with a smaller ingoing diameter engaging one or more portions of the tape, such as the tape edges, in one leg and a larger outgoing diameter contacting at least one portion of the tape, such as the tape center, in the other leg of the tape path.
Ideally, this method would have allowed reel tensions to be dimensioned optimally in terms of the tape pack. In practice, however, such an approach engenders differential tape tensions across the tape and practically requires a dampening of the idler roller at the bight portion of the U-shaped tape path. Even after such modification, the dual diameter capstan system was limited in its use to unidirectional tape drives or then depended on the use of pinch rollers to press different tape portions against the different diameter capstan portions for different directions of tape advance.
In a modified closed loop approach, the idler at the bight portion of the U-shaped tape path was replaced by an active roller slaved to the motor-driven capstan between the tape legs by a drive belt. In a sense, there were thus two capstans in the tape loop driven by one capstan motor with the aid of a drive belt interconnecting the two capstans.
In practice, such a system has an inherent double-ended limitation. If the drive belt coupling the two capstans is made pliable in the nature of a rubber band, resulting speed fluctuations between the two capstans severely limit servo bandwidth of the system. On the other hand, if the drive belt is made of stiff material, it introduces a self-resonance which relegates attainable servo bandwidths to regions below the particular self-resonance.